1. Field of the Invention
The present invention relates generally to an optical information recording medium in which information is recorded and reproduced optically with respect to recording layers in a multilayer, i.e. an optical multilayer disk, and to a method of recording and reproducing with respect to the same. The present invention also relates generally to an optical waveguide device, to which a coherent light source is applied, used in fields of optical information processing and optical application instrumentation and further to a multiwavelength light source using such an optical waveguide device and an optical system using such a multiwavelength light source for recording and reproduction of information with respect to the optical multilayer disk.
2. Related Background Art
As a method of increasing capacity and density of an optical information recording medium, a technique of recording using two wavelengths with respect to two recording layers is proposed by H. A. Wierenga in “Phase Change Recording: Options for 10−20 GB (Dual Layer, High NA, and Blue)” (Proc. SPIE. Optical Data Storage '98, 3401, 64–70 (1998)), in which dual-wavelength recordings using wavelengths of 650 nm and 780 nm and wavelengths of 410 nm and 650 nm have been reported based on calculation.
In this case, the spot size w of an incident beam is given by w=kλ/NA. In the formula, λ denotes the wavelength of a laser used, k a constant, and NA a numerical aperture of an objective lens. From this relation, the spot size can be reduced and recording density is increased as the wavelength λ of the laser beam is shortened and the numerical aperture NA of the lens is increased.
FIG. 11 is a schematic view illustrating a recording and reproduction method with respect to a conventional dual-layer optical information recording medium. Viewed from the side on which a laser beam 25 is incident, a first recording layer and a second recording layer are formed. A multilayer structure including the first recording layer is a first recording medium 17 and that including the second recording layer is a second recording medium 18. A first substrate 21 with the first recording medium 17 formed thereon and a second substrate 22 with the second recording medium 18 formed thereon are bonded with an adhesion layer 15, thus obtaining a dual-layer optical information recording medium. With respect to both the first recording medium 17 and the second recording medium 18, recording and reproduction are performed using a laser beam 25 with a wavelength λ. In the figure, R1 indicates a reflectance of the first recording medium 17 with respect to the wavelength λ and R2 denotes a reflectance of the second recording medium 18 with respect to the wavelength λ.
FIG. 12 is a structural view of a system according to the conventional recording and reproduction method with respect to an optical information recording medium. In order to carry out excellent recording and reproduction with respect to both the first and second recording media, the light absorption ratios of both the recording layers and the light transmittance of the first recording medium with respect to the wavelength λ must satisfy predetermined conditions. The light absorption ratio denotes Ac/Aa, where Ac (%) indicates the light absorptance of the recording layers in a crystal state and Aa (%) indicates the light absorptance of the recording layers in an amorphous state. For example, JP 2094839 B discloses that in order to secure an excellent erase rate, it is important to adjust the rising rates of temperature in the crystal state and in the amorphous state to be the same and therefore, it is necessary to satisfy the relationship of Ac/Aa≧1.0.
When the light transmittance of the first recording medium with the first recording layer being in the crystal state is indicated as Tc (%) and the light transmittance of the first recording medium with the first recording layer in the amorphous state as Ta (%), higher light transmittances Tc and Ta are desirable since the second recording medium is recorded or reproduced with a laser beam that has passed through the first recording medium. On the other hand, when the light transmittances Tc and Ta are too high, in view of the distribution of an incident beam, the light absorptances Aa and Ac decrease, thus causing difficulty in recording in the first recording medium.
From the recording experiment using a laser wavelength in the vicinity of 660 nm conducted by the present inventors, it was found that in order to obtain excellent recording and reproduction characteristics in both the first and second recording media, it is preferable that conditions of Tc≧45 and Ta≧45 are satisfied.
In this connection, optical characteristics such as optical reflectance R and light transmittance T in a recording medium with a multilayer structure to be recorded and reproduced and light absorptance A of the respective layers with respect to a wavelength λ can be calculated precisely by, for example, a matrix method (for example, see Chapter 3 in “Wave Optics” by Hiroshi Kubota, published by Iwanami Shinsho, 1971) when complex refractive indexes (a refractive index and an extinction coefficient) of the respective layers with respect to the wavelength are known. Therefore, the wavelength dependence of the complex refractive indexes of the respective layers is a key factor for determining the optical characteristics of a multilayer structure.
According to the experiments conducted by the present inventors, in dual-layer recording using a laser wavelength in the vicinity of 660 nm, both the relationships of light absorption ratio≧1.0 and light transmittance≧45% are satisfied, thus obtaining excellent recording and reproduction characteristics in both the first and second recording media.
In the dual-wavelength recording proposed by Wierenga, however, laser beams with wavelengths of 410 nm (blue) and 650 nm (red) are used, and therefore, at least two optical heads are required. In addition, one of the wavelengths is 650 nm, which is long and thus there still is room left for the increase in recording density.
According to optical calculation conducted by the present inventors, due to the high wavelength dependence of the complex refractive indexes of the recording layers, the light absorptance ratio of the first recording layer in the vicinity of a wavelength of 400 nm was no more than 1.0, and thus, the difficulty in satisfying the condition of the light transmittance≧45% in the first recording medium simultaneously was found. In this case, the first recording medium has an insufficient erase rate, or sufficient laser beams do not reach the second recording medium, thus causing a lack of recording power with respect to the second recording medium.
In recording and reproduction of information with respect to such an optical information recording medium, a multiwavelength light source capable of emitting laser beams with a plurality of different wavelengths is used. For instance, such laser beams with a plurality of different wavelengths can be obtained by optical wavelength conversion.
Optical wavelength conversion utilizing a nonlinear optical effect has been applied to various fields to achieve a reduction in wavelength and an increase in operating wavelength range. Particularly, second harmonic generation (SHG) and sum frequency generation (SFG) utilizing a secondary-nonlinear optical effect are effective means for obtaining a short-wavelength light source, and various light sources have been practically used. Among others, in an optical-waveguide nonlinear optical device utilizing an optical waveguide, its efficiency can be improved easily, and it is possible to reduce its size and to provide mass-productivity by a wafer process. Thus, it is expected that the device will be applied to consumer products as a small short-wavelength light source.
Currently, an optical waveguide SHG element generally used is a quasi-phase-matching (QPM) SHG element using a polarization inversion structure having a periodicity. The QPM-SHG element has advantages such that a phase matching wavelength can be set arbitrarily by means of a polarization inversion period, wavelengths can be converted with high efficiency, and the like, which enable optical waveguides for different phase matching wavelengths to be formed in one element. Conventionally, QPM-SHG elements using this have been proposed.
FIG. 13 shows a plan view illustrating an example of a conventional optical waveguide device in which optical waveguides having different phase matching characteristics are integrated on one substrate. In FIG. 13, a plurality of optical waveguides 132 are formed on a LiNbO3 substrate 131 and a polarization inversion structure 133 with different periods is formed so as to traverse the optical waveguides 132, thus forming a plurality of optical waveguides having different phase matching characteristics on one substrate. As a disadvantage of the QPM-SHG element, there is a problem that the tolerance with respect to phase matching wavelengths is extremely narrow. In this element, optical waveguides allowing phase matching wavelengths to be gradually different are formed, which permits optical waveguides for phase matching with the wavelength of a fundamental light source to be formed in any location. In other words, the selection of a suitable optical waveguide enables the phase matching with the fundamental wave with an arbitrary wavelength.
FIG. 14 is a plan view illustrating a conventional QPM-SHG element, in which the tolerance with respect to the phase matching wavelength is increased, as an optical waveguide device with optical waveguides for different phase matching wavelengths integrated on a substrate. In FIG. 14, one optical waveguide 142 is formed on a substrate 141 and a plurality of polarization inversion regions Λ1, Λ2, and Λ3 are formed on the optical waveguide 142 as a polarization inversion structure 143. The respective polarization inversion regions have different phase matching conditions. With the combination of the polarization inversion regions with the different phase matching conditions, it is intended to increase the tolerance with respect to the phase matching wavelength in the optical waveguide device as a whole. By the increase in the tolerance with respect to the wavelength, stable output characteristics to the variations in wavelengths of fundamental waves can be obtained.
On the other hand, it also has been proposed to obtain a light source with a plurality of wavelengths using a semiconductor laser. There is a method including forming different active layers on a semiconductor laser and emitting laser beams with different wavelengths from one chip.
The present invention is intended to achieve a configuration for obtaining a plurality of coherent beams with different wavelengths from a single emission point or adjacent emission points using an optical waveguide device.
On the other hand, in a conventional waveguide optical device, a plurality of optical waveguides with different phase matching characteristics are integrated on one device, but the configuration for simultaneous conversion of different wavelengths of fundamental waves has not been proposed.
Furthermore, there has been a configuration for emitting beams with different wavelengths simultaneously from a semiconductor laser of one chip. However, since emission parts of optical waveguides are formed at different positions, when two emission beams are intended to be focused simultaneously with one optical system, great aberration is caused and a complicated optical system is provided for obtaining focusing characteristics within the diffraction limit, which have been problems.